Load or force sensors typically use a pair of facing plastic sheets on which conductive areas or electrodes are printed. An example of such a structure is disclosed in U.S. Pat. No. 4,856,993 to Maness et al. A pressure-sensitive resistive material is located between the two confronting electrodes or conductive areas. When force is applied at confronting electrodes, the pressure-sensitive resistive material reduces in resistance so that current will flow as a function of the force applied. The pressure-sensitive resistive material may thus be described as semiconductive. The change in resistance and current flow may be sensed and utilized by suitable instrumentation described in Maness et al U.S. Pat. No. 4,856,993 and known to the art.
Typically, force sensors are carefully printed to very close tolerances and require careful overprinting of the semiconductive material as well. Conductive electrodes may be disposed as crossing grids as shown by U.S. Pat. No. 4,856,993. Another pattern uses a button-like conductive area on each of two plastic sheets with each of the conductive areas being connected to a lead also patterned onto the corresponding plastic sheet. A pattern of semiconductive material corresponding to the conductive area is printed on one or both of the conductive areas. Adhesive is used to hold the two sheets together, and the resulting adhesive layer is disposed around the pattern of the conductive and semiconductive materials and outside of the sensing zones of the force sensor.
The above described force sensors and related methods for making them suffer from various drawbacks and disadvantages. For example, the need to selectively print a pattern of conductive material onto the plastic sheets, as by silk-screening, is not only time consuming, but also requires substantial precision in the printing, irrespective of what pattern is being applied. In addition, since the electrode patterns on the confronting surfaces of the sheets are generally somewhat different from each other, a separate, specifically designed screen or tooling is required for each of the patterns. Yet another specifically designed screen is also required for applying the semiconductive material in the appropriate pattern.
The required precision, tooling and selective printing operations make manufacturing complicated and costly. The required precision in printing may often lead to an undesirably high percentage of scrap material being generated during the process of making force sensors according to prior art techniques.
There is thus a need for a force sensor of a simple, yet effective, construction and made by a simplified method.